Optical filter module, and manufacturing method thereof

ABSTRACT

An optical filter module having a multilayered filter inserted in the paths of optical waveguides thereof. In the optical filter module, precise V-shaped grooves are formed on the principle plane of a plane substrate so that optical fibers protrude therefrom. The optical fibers are mounted on the plane substrate. After the multilayered filter is inserted in a filter insertion groove that is formed so as to intersect the V-shaped grooves on the plane substrate, the filter is sandwiched and clamped by two sheets of covers provided on the plane substrate. Provided is a method of precisely mounting the filter that improves the optical axis adjustment between input and output optical fibers and reduces misregistration of the filter to be inserted.

TECHINICAL FIELD

The present invention relates to an optical filter module for opticalcommunication that incorporates various kinds of optical filters thereinand to a method of manufacturing the optical filter module.

BACKGOUND ART

In an optical information communication system using optical fibers,light is attenuated by being radiated from or absorbed by the opticalfibers. Used as a system for amplifying this attenuated light is anoptical fiber amplifier. Being capable of amplifying the output withoutconverting optical signals into electrical signals, the optical fiberamplifier is a system essential to an optical communication network.FIG. 8A shows an example of the structure of the system. An opticalfiber amplifier is made of optical isolator 53, erbium-doped opticalfiber 51, pumping laser 54 used as laser light for excitation,photodiode 52 for monitoring output, and filter 55 for adjusting gains.As pumping laser 54 for a single-mode fiber, laser light having awavelength of 1.48 μm is used.

The amplification gains of erbium-doped optical fiber 51 are dependenton frequencies, as shown by characteristics A of FIG. 8B. Therefore, itis necessary to insert filter 55 having characteristics B, i.e. theinverse of characteristics A, into the transmission path after theamplification of optical output and flatten the amplification gains asshown by characteristics C. Filter 55 is called a gain-flatteningfilter. Generally, filter 55 is used as a part of a module thatincorporates the filter inserted between optical fibers. FIG. 8C showsthe structure of the module. In the module, optical fibers 56,collimating and collective lenses 57, and gain-flattening filter 58 arehoused and sealed into metal housing 59 after optical axis adjustment.

In the optical passage connection method and the filter insertion methodin the conventional module using lenses described above, the lensesthemselves have certain sizes, and distances between the opticalpassages and the lenses necessary for focusing are required. Therefore,the size reduction of the entire module is restricted to a certaindegree.

Additionally, each of the optical passages must be connected accurately.For a module using lenses, adjustment of the lenses requires cost andtime: thus, productivity of the module is poor.

Proposed as a method of addressing the problems described above is theJapanese Patent No. 3175814. FIGS. 9A and 9B are drawings showing thestructure of the invention. The invention is an example of a reflectiontype optical multiplexer/demultiplexer for wavelength divisionmultiplexing (WDM) in which a filter insertion groove is formed so as tointersect embedded optical waveguides formed on a silicon substrate, anda multilayered filter is inserted and integrated in the groove.

Passages for guiding the waves of light are formed in one surface ofclad material 32 on silicon substrate 31 as waveguides. Filter insertiongroove 36 is formed so as to intersect the waveguides. This structureeliminates the need of adjustment of optical axes of input waveguide 33and output waveguides 35 and 34. Further, width Dg of filter insertiongroove 36 and radius of curvature R of the warp of multilayered filer 37(hereinafter referred to as a “filter”) are set so as to satisfy thefollowing relation:R<W ²/8(Dg−Df)where the lateral width of filter 37 is W, and the thickness thereof isDf. This relation proposes a structure in which filter 37 can securelybe fitted in filter insertion groove 36 utilizing the warp of filter 37.In other words, proposed is a module structure in which embedded opticalwaveguides form optical waveguide paths, and various kinds of filtersare inserted and integrated in the paths.

However, in this system, filter 37 has a warp having at least a certainradius of curvature. Therefore, when the filter isn't mounted at theexactly correct position, the angle of incidence to filter 37 is largelydisplaced. Further, when embedded waveguides are used as opticalwaveguide paths, the warp of multilayered filter 37 inevitably generatesa gap between the multilayered filter and waveguides 35 and 33 in filterinsertion groove 36. This gap causes scattering and loss of light fromwaveguides 35 and 33. Additionally, it is extremely difficult and thusdisadvantageous for production to mechanically hold and insert a warpedcomponent in a microgroove.

DISCLOSURE OF THE INVENTION

Proposed is an optical filter module in which a filter insertion grooveis formed so as to intersect optical passages on a plane substrate, afilter is inserted in this filter insertion groove, and covers cover theoptical passages so as to be bonded onto the plane substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view illustrating a structure of an optical filtermodule in accordance with a first exemplary embodiment of the presentinvention.

FIG. 1B is a side view of the optical filter module in accordance withthe first exemplary embodiment of the present invention.

FIG. 1C is a sectional view of the optical filter module in accordancewith the first exemplary embodiment of the present invention.

FIGS. 2A through 2D are sectional views each illustrating a shape of afilter insertion groove in the structure of the optical filter module inaccordance with the first exemplary embodiment of the present invention.

FIG. 3A is a top view illustrating an example of a structure of anoptical filter module in accordance with a second exemplary embodimentof the present invention.

FIG. 3B is a side view of the optical filter module in accordance withthe second exemplary embodiment of the present invention.

FIG. 3C is a sectional view of the optical filter module in accordancewith the second exemplary embodiment of the present invention.

FIGS. 4A through 4G are explanatory views illustrating a method ofmanufacturing an optical filter module in accordance with a thirdexemplary embodiment of the present invention.

FIGS. 5A through 5G are explanatory views illustrating a method ofmanufacturing an optical filter module in accordance with a fourthexemplary embodiment of the present invention.

FIGS. 6A through 6F are explanatory views illustrating a method ofmanufacturing an optical filter module in accordance with a fifthexemplary embodiment of the present invention.

FIGS. 7A through 7F are explanatory views illustrating a method ofmanufacturing an optical filter module in accordance with a sixthexemplary embodiment of the present invention.

FIG. 8A is a drawing illustrating a structure of a conventional exampleof an optical amplifying system.

FIG. 8B is a drawing showing characteristics of the conventional exampleof the optical amplifying system.

FIG. 8C is a sectional view of a conventional optical filter module.

FIG. 9A is an explanatory view illustrating how to mount a conventionaloptical filter.

FIG. 9B is an enlarged view of an essential part of FIG. 9A.

DETAILED DESCRITION OF PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are described hereinafterwith reference to the accompanying drawings. Same elements are denotedwith the same reference numbers, and their detailed descriptions areomitted.

First Exemplary Embodiment

FIG. 1A shows a top view of an optical filter module having two opticalpasses in accordance with a first exemplary embodiment of the presentinvention. FIG. 1B shows a side view thereof. FIG. 1C shows a sectionalview thereof. Filter 2 is inserted in a filter insertion groove 1(hereinafter referred to as a “groove”), and bonded by optical adhesive6. Each of optical fibers 4 for forming optical passages is provided ina space that is formed by V-shaped groove 10, formed on the principlesurface of plane substrate 5, and covers 3.

In production of an optical filter module, optical passages are notnecessarily integrated with plane substrate 5. This renders greatergeneral versatility to materials of the optical passages and planesubstrate. When an optical passage that is not integrated with planesubstrate 5, such as optical fiber 4, is used, precise V-shaped groove10 is formed on plane substrate 5 for fixation of optical fiber 4. TheV-shaped groove is used as a guide for fixation of optical fiber 4.Production of filter insertion groove 1 so as to intersect V-shapedgrooves 10 facilitates insertion and connection of filter 2, withoutcausing misalignment of the optical axes of optical fibers 4. Further,the extremely simple structure allows downsizing of the module andimprovement of productivity thereof.

Filter 2 is formed by laminating a large number of dielectric thin filmsmade of a material, e.g. SiO₂ and Ta₂O₅, on a resin substrate made of amaterial, e.g. glass and polyimide.

Covers 3 and plane substrate 5 are formed of glass, silicon, or othermaterials. Desirably, the covers and plane substrate have linearexpansion coefficients approximating to that of optical fibers 4.However, when a resin-based adhesive (not shown) is used for bondingthem, the elasticity of the adhesive can alleviate the stress caused bythe difference in liner expansion coefficient. When a photo-curingadhesive is used, it is desirable that the covers and plane substrateare made of optically transparent materials.

In order to prevent light from scattering from the end faces of opticalfibers 4, an optical adhesive having a refractive index substantiallyequal to that of a material used for optical fibers 4 is used as opticaladhesive 6. When optical fibers 4 made of fused silica are used, it ispreferable to use optical adhesive 6 having a refractive indexapproximating to 1.44, i.e. the refractive index of fused silica.

Formed on plane substrate 5 are V-shaped grooves 10 for mounting andfixing optical fibers 4. Machining V-shaped grooves 10 so as to have adepth and angle of predetermined values allows the optical fibers 4 tobe sandwiched by covers 3 and plane substrate 5 and mounted withoutmisregistration. V-shaped grooves 10 can also be formed on covers 3 inaddition to plane substrate 5. Setting the following equation:d=r/sin(α/2)allows the center of optical fiber 4 to be placed on the surfaces ofcover 3 and plane substrate 5, where the point angle of each of V-shapedgrooves 10 on plane substrate 5 and cover 3 is α the radius of opticalfiber 4 to be mounted is r, and the depth of each V-shaped groove 10 isd. Generally, the diameter of a clad, i.e. the optical passage of aglass-based optical fiber is 125 μm. Thus, setting the point angle ofV-shaped groove 10 to 90 degrees and the depth thereof to approx. 180μm, and sandwiching fiber 4 between cover 3 and plane substrate 5 cansecurely clamp optical fiber 4. V-shaped grooves 10 formed on covers 3and plane substrate 5 need not have the same depth and anglenecessarily. The angle and depth of V-shaped groove 10 formed on cover 3are set to values equal to or larger than those of a portion of opticalfiber 4 projecting from the principle surface of plane substrate 5, whenoptical fiber 4 is mounted in V-shaped groove 10 on plane substrate 5.Thus, cover 3 and plane substrate 5 allow the optical fiber to bemounted and clamped securely.

Groove 1 in plane substrate 5 is formed to have a predetermined angle ofθ with respect to optical fibers 4. This is for preventing lightincident from optical fibers 4 upon filter 2 from reflecting. The tiltangle depends on the wavelength of light to be guided. For example, whenlight having a wavelength of 1.48 μm is guided using a single-modefiber, it is desirable to provide a tilt angle θ ranging from approx. 5to 10 degrees. For a tilt angle θ smaller than 5 degrees, reflection hasgreater influence. For a tilt angle θ exceeding 10 degrees, refractionmakes it difficult to set the optical axes, and affects thecharacteristics of filter 2.

In this embodiment, a groove is formed across continuous lengths ofoptical passages and a filter is inserted therein. This structuresimplifies adjustment of the optical axes of the optical passages.Further, eliminating the need of using lenses simplifies the structure.As a result, the entire optical filter module is downsized.

The use of a core diameter expanding fiber having a partially expandedcore in which light in optical fiber 4 is enclosed and guided furtherreduces the loss caused by connection of optical fiber 4 and filter 2.

The optical filter module of this embodiment is structured so thatgroove 1 is formed so as to intersect optical passages 4 and filter 2 isinserted therein. Because of this structure, optical fibers 4 are cut bygroove 1, and gap G having a width of groove 1 exists between theendfaces of optical fibers 4 via filter 2. When spot size W of light ateach of the endfaces of optical fibers 4 is the same, power transmissioncoefficient Tg between the endfaces of optical fibers 4 via gap length Gis represented by the following equation:Tg=[1+(λ×G/(2×π×n×W ²))²]⁻¹where the refractive index of multilayered filter 2 is n, and λrepresents a wavelength. This equation indicates that the transmissionloss increases in proportion to increases in wavelength and gap length Gand rapidly decreases in proportion to the inverse number of the squareof a spot size.

As a result, the larger spot size at each of the endfaces of opticalfibers 4 exposed to the wall surface of groove 1 is advantageous forreducing the transmission loss. As a core diameter expanding fiber,various kinds of types are proposed. Typical examples include athermally expanded core (TEC) fiber. This optical fiber is a special onein which a doped element for controlling the refractive index of thecore, such as GeO₂, is diffused to a part of the clad by heating a partof the clad, to substantially expand the core diameter. The use of thiskind of optical fiber increases the spot size of light emitted from eachendface of optical fibers 4. When a core diameter expanding fiber isused for the optical filter module of the present invention, groove 1 ismachined and formed so as to intersect the largest portion of the corediameter expanding part in each optical passage 4.

As a result, the larger spot size at each of the endfaces of opticalfibers 4 exposed to the wall surface of groove 1 is advantageous forreducing the transmission loss. As a core diameter expanding fiber,various kinds of types are proposed. Typical examples include athermally expanded core (TEC) fiber. This optical fiber is a special onein which a doped element for controlling the refractive index of thecore, such as GeO₂, is diffused to a part of the clad by heating a partof the clad, to substantially expand the core diameter. The use of thiskind of optical fiber increases the spot size of light emitted from eachendface of optical fibers 4. When a core diameter expanding fiber isused for the optical filter module of the present invention, groove 1 ismachined and formed so as to intersect the largest portion of the corediameter expanding part in each optical passage 4.

Various shapes shown in FIGS. 2A through 2D are applicable as the shapeof groove 1. The bottom of groove 1 shown in FIG. 2A is shaped to beshallower on the sides of both groove walls, and have a flat portion atthe center of the bottom of which central portion is deeper at an angleof θ1. FIG. 2B shows a case where groove 1 has a certain angle of θ2from the bottom to the principle surface of plane substrate 5 on theside of one of groove walls. FIG. 2C shows a case where a part of thebottom of groove 1 has a flat portion on the side of one of groovewalls, and the bottom has a certain angle of θ3 from the flat portion tothe side of the other groove wall. In FIG. 2D, the bottom of groove 1 isshaped semi-circular.

Each of these grooves 1 is formed using a diamond grindstone having atip previously formed into the shape of the groove, for example. Becausegroove 1 is formed by transferring the shape of the tip of the diamondgrindstone to the groove shape, the groove shape after machining largelydepends on the forming accuracy of the diamond grindstone. When thewidth of a grindstone is up to 0.1 mm, machining is difficult becausethe diameters of usable diamond particles are restricted. However, whenthe width of the grindstone exceeds 0.1 mm, machining the shapes isrelatively easy.

As shown in FIGS. 2A through 2C, groove 1 is machined to irregularshapes so that the tilt angle formed by one groove wall with the otheris small. The reason for machining in this manner is to prevent filter 2from tilting after insertion. When groove 1 is formed using the endfaceof cover 3 previously mounted as a reference surface, force is exertedin the direction in which the diamond grindstone for machining is forcedtoward cover 3, i.e. the reference surface, during machining. In orderto prevent deterioration of machining accuracy of groove 1 caused bymeander of the diamond grindstone, such machining is effective.

Tapering the bottom of groove 1 from both groove walls to the bottomsurface as shown in FIG. 2A prevents inserted filter 2 from vacillatingand being fixed at a different angle with respect to the optical axes.

Groove 1 shown in FIG. 2B has the effects equivalent to those of groove1 shown in FIG. 2A. Additionally, forming the section of groove 1 into aV shape as shown in FIG. 2B allows filter 2 that is even extremely thinfor groove 1 to be fixed securely as long as it is inserted as far as itgoes. In other words, the V-shaped groove broadens the allowable rangeof the thickness of filter 2 with respect to groove 1.

Groove 1 shown in FIG. 2D has the effects equivalent to those of groove1 shown in FIG. 2A. Additionally, forming the bottom surface of groove 1semicircular as shown in FIG. 2D facilitates machining in production ofgroove 1. Further, because the bottom surface is curved, the angle atwhich filter 2 is erected in groove 1 can freely be set in the range ofthe width of the groove.

Groove 1 shown in FIG. 2C has the effects equivalent to those of groove1 shown in FIG. 2A. Additionally, groove 1 as shown in FIG. 2C has abottom tapered from one of groove walls to the other. Because one groovewall is not tapered, filter 2 can be fixed along the surface. Further,even when optical fibers 4 and covers 3 are attached before machining ofthe groove, groove 1of such a shape can be formed without a bit hittingcovers 3 or displaced from the endfaces of the covers during machining.

In the first exemplary embodiment, the optical filter module has twooptical fibers. However, an optical filter module having one opticalpassage can be fabricated. Inversely, it is also possible to provide afilter insertion groove in a place where a plurality of optical passagesintersect or a plurality of optical passages are disposed in parallelwith each other, so as to intersect the optical passages, and insert afilter in the groove. Therefore, the effects of the filter can be sharedby a plurality of optical passages, and thus a more multi-functionaloptical filter module can be fabricated.

Second Exemplary Embodiment

FIG. 3A shows a top view of an optical filter module in accordance witha second exemplary embodiment. FIG. 3B shows a side view thereof. FIG.3C shows a sectional view thereof. What is different from the firstexemplary embodiment is described hereinafter. In FIG. 1A, a minuteangle θ is provided in order to prevent scattering and reflection causedby filter insertion groove 1. However, in FIG. 3A, this minute angle θis not provided. Instead, a minute angle θ is provided in groove 1 inthe side view of FIG. 3B. For this reason, in order to preventdeterioration of machining accuracy, it is desirable to have a groovestructured as shown in FIG. 2B or 2C. In every respect other thandescribed above, this embodiment has a structure and effects similar tothose of the first exemplary embodiment. Similarly, a moremulti-functional optical filter module having two or more opticalpassages can also be fabricated.

Third Exemplary Embodiment

Described hereinafter is a method of fabricating a structure of anoptical filter module. FIGS. 4A through 4G are sectional viewssequentially showing a procedure of fabricating an optical filter moduleof the present invention.

As a first step, as shown in FIG. 4B, a necessary number of V-shapedgrooves 10 for fixing optical fibers 4 forming optical passages areprecisely formed on plane substrate 5 shown in FIG. 4A. Next, as asecond step, filter insertion groove 1 sufficiently wide for thethickness of filter 2 is formed so as to intersect these preciseV-shaped grooves 10. At this time, as shown in the first and secondexemplary embodiments, groove 1 is formed so as to be displaced at aminute angle of θ from the right angle of the optical passages.Specifically, θ ranges from 5 to 10 degrees, and is changed according tothe wavelength of light to be used. Placing filter 2 along the wallsurface of groove 1 formed at an arbitrary angle required allows filter2 to be fixed at the arbitrary angle designed. Groove 1 is formed tohave an appropriate shape selected from the structures shown in FIGS. 2Athrough 2D.

For example, groove 1 is formed by grinding with a grindstone shaped tothe groove using cubic boron nitride or diamond as abrasive grains, ormachining using particles, such as blasting. When silicon is used as amaterial of plane substrate 5, wet etching or dry etching can also beused. When groove 1 is machined to have an irregular sectional shape,the use of a formed grindstone allows production of the irregular shapesmore freely and thus is more advantageous.

As a third step, as shown in FIG. 4C, each optical fiber 4 is installedin corresponding V-shaped groove 10. At this time, it is desirable thatthe portion of mounted optical fiber 4 is a clad portion with coatingremoved, in consideration with mounting accuracy. Additionally, theangle and depth of each V-shaped groove 10 are set so that each opticalfiber 4 is flush with the principle surface of plane substrate 5. Theuse of core diameter expanding fibers as optical fibers 4 can increasethe spot size of each of the endfaces of optical fibers 4 and decreasethe transmission loss and connection loss between optical fibers 4. Atthis time, optical fibers 4 are installed so that the core diameterthereof is largest in groove 1.

Next, as a fourth step, as shown in FIG. 4D, optical adhesive 6 isapplied to the entire principle surface of plane substrate 5. Further,as shown FIG. 4E, filter 2 is inserted in groove 1 having opticaladhesive 6 charged therein. At this time, the size of filter 2 is set sothat the upper portion of filter 2 always projects from the principlesurface of plane substrate 5. Additionally, the inside of groove 1formed in the second step is also filled with optical adhesive 6. Theuse of optical adhesive 6 having a refractive index substantially equalto that of the core material of optical fibers 4 to be used can decreasethe connection loss between the optical fibers. Methods of curingoptical adhesive 6 include using light, e.g. ultraviolet light, andheating. The method of using light is advantageous for reducingproduction time.

Next, as a fifth step, as shown in FIG. 4F, covers 3 are moved from bothsides of filter 2 projecting from groove 1 to sandwich and clamp thefilter on both sides. At this time, mounting one of covers 3 so that theside face thereof is aligned with one of the side faces of groove 1allows filter 2 to be mounted and fixed along the side face of groove 1.This method eliminates the need of registering two sheets of covers 3 atthe same time and thus simplifies fabrication of the optical filtermodule. Additionally, when filter 2 is clamped at an angle, fixing oneof the covers makes it easy to adjust the angle. This method is alsouseful to prevent breakage of filter 2 when the filter is sandwiched bythe other cover 3. The sandwiched portion is a projecting portion offilter 2. It is desirable that the length of the projecting portion offilter 2 is at least half the thickness of cover 3 in consideration ofthe strength of filter 2, because filter 2 is sandwiched by covers 3.When filter 2 is warped, the projecting portion is made longer toincrease the area sandwiched by covers 3: thus, the warp is straitened.When glass or other material is used for the substrate of filter 2, itslarge elastic modulus is effective in preventing breakage caused bystress.

At last, as a sixth step, as shown in FIG. 4G, the optical filter moduleis completed by irradiating covers 3 and/or plane substrate 5 withlight, such as ultraviolet light, or heating, to cure the adhesive. Theuse of a photo-curing adhesive or heat-hardening adhesive as adhesive 6allows covers 3 to be bonded without application of mechanical pressure.Thus, the optical axes of optical fibers 4 are unlikely to bemisaligned. Additionally, the time taken for mounting can be reduced.

Fourth Exemplary Embodiment

FIGS. 5A through 5G show a procedure of fixing optical fibers 4 on planesubstrate 5, and then forming filter insertion groove 1 in planesubstrate 5 and inserting filter 2 therein.

As a first step, as shown in FIG. 5A, a necessary number of V-shapedgrooves for fixing optical fibers 4 are precisely formed on theprinciple surface of plane substrate 5. At this time, the angle anddepth of each V-shaped groove 10 are set so that each optical fiber 4 isflush with the principle surface of plane substrate 5.

As a second step, as shown in FIG. 5B, each optical fiber 4 is installedin corresponding V-shaped groove 10. The use of core diameter expandingfibers as optical fibers 4 is effective in increasing the spot size ofeach of the endfaces of optical fibers 4 and decreasing the transmissionloss and connection loss between optical fibers 4. At this time, eachoptical fiber 4 is installed so that the core diameter thereof islargest in a place where groove 1 is to be formed.

As a third step, as shown FIG. 5C, groove 1 sufficiently wide for thethickness of filter 2 is formed so as to be displaced at a minute angleof θ from the right angle of optical fibers 4. In this case, opticalfibers 4 are cut at the same time. In order to decrease surfaceroughness of the endfaces of cut and separated optical fibers 4, it isdesirable to use abrasive grains as small as possible in machining. Inthe present invention, a diamond grindstone is used to form groove 1.The endfaces of optical fibers 4 like a mirror surface can be obtainedby selecting diamond abrasive grains of at least #3000 (average particlediameter: up to 5 μm).

The steps shown in FIGS. 5D through 5G are similar to those of the thirdexemplary embodiment, and thus the descriptions are omitted.

Fifth Exemplary Embodiment

FIGS. 6A through 6F show a procedure of fixing optical fibers 4 at oneof the endfaces of a groove, inserting filter 2 therein, and thenplacing the other optical fibers 4 against the filter.

As a first step, as shown in FIG. 6A, a necessary number of V-shapedgrooves 10 for fixing optical fibers 4 are precisely formed on theprinciple surface of plane substrate 5. At this time, the angle anddepth of each V-shaped groove 10 are set so that each optical fiber 4 isflush with the principle surface of plane substrate 5.

As a second step, as shown in FIG. 6B, filter insertion groove 1sufficiently wide for the thickness of filter 2 is formed so as tointersect V-shaped grooves 10. At this time, as shown in the first andsecond exemplary embodiments, groove 1 is formed so as to be displacedat a minute angle of θ from the right angle of the optical passages. Asgroove 1, an appropriate shape is selected from the structures shown inFIGS. 2A through 2D.

As a third step, as shown in FIG. 6C, optical adhesive 6 is applied tothe entire principle surface of plane substrate 5. Either optical fibers4 for input or optical fibers 4 for output is placed in correspondingV-shaped grooves 10.

As a fourth step, as shown in FIG. 6D, filter 2 is inserted in groove 1.Then, the other optical fibers 4 are inserted as they are slid alongcorresponding V-shaped grooves 10. The other optical fibers are placedagainst filter 2 so that the gap between filter 2 and the other opticalfibers 4 is as small as possible. This placement can considerablydecrease attenuation of light caused by scattering, irregular reflectionand changes in refractive index. At this time, the use of core diameterexpanding fibers as optical fibers 4 is effective in increasing the spotsize of each of the endfaces of optical fibers 4 and decreasing thetransmission loss and connection loss between optical fibers 4. Eachoptical fiber 4 is installed so that the core diameter thereof islargest in groove 1.

The steps shown in FIGS. 6E and 6F are similar to those of the thirdexemplary embodiment, and thus the descriptions are omitted.

Sixth Exemplary Embodiment

FIGS. 7A through 7F show a procedure followed when optical passages 20have already been embedded in plane substrate 5.

When optical passages 20, such as diffusion glass waveguides and polymerwaveguides, are embedded in plane substrate 5, as shown FIG. 7A,V-shaped grooves 10 need not be formed. As shown FIG. 7B, groove 1sufficiently wide for the thickness of filter 2 is formed so as to bedisplaced at a minute angle of θ from the right angle of the opticalpassages. Next, adhesive 6 is applied to the top surface of planesubstrate 5 as shown in FIG. 7C, and filter 2 is inserted therein asshown in FIG. 7D. At this time, adhesive 6 having a refractive index asequal as possible to that of optical passages 20 is selected. Afterinsertion of filter 2 in groove 1, two sheets of covers 3 placed onplane substrate 5 are moved as shown in FIG. 7E, to sandwich filter 2.At this time, filter 2 is clamped at a predetermined angle. As shownFIG. 7F, filter 2 is sandwiched at a desired angle and bonded usinglight or heat.

INDUSTRIAL APPLICABILITY

Fixing optical passages using precise V-shaped grooves so as to avoidmisalignment of optical axes thereof facilitates optical axisadjustment, which is supposed to be most critical and difficult inconnection of optical passages. Forming a filter insertion groove so asto be displaced at a minute angle of θ from the right angle of theoptical axes can prevent entry of reflected or scattered light caused bythe wall surfaces of the groove. Selecting a filter insertion groovehaving a shape most appropriate for a filter to be inserted allows moreprecise connection between the filter and the optical passages. Inproduction of an optical filter module, it is difficult to insert afilter while precisely aligning optical axes. There are also problems indownsizing. This structure can provide an extremely small optical filtermodule easily.

1. An optical filter module comprising: an optical passage; a planesubstrate fabricated to form a filter insertion groove thereon so as tointersect said optical passage; a filter inserted in said filterinsertion groove; and covers covering said optical passage and bonded tosaid plane substrate, said covers sandwiching and clamping a portion ofsaid filter projecting from said filter insertion groove therebetween,wherein a length of the portion of said filter projecting from saidfilter insertion groove is at least half the thickness of each of saidcovers.
 2. The optical filter module of claim 1, wherein said opticalpassage has an intersection; and said filter insertion groove intersectssaid intersection.
 3. The optical filter module of claim 1, wherein saidoptical passage is an optical fiber.
 4. The optical filter module ofclaim 1, wherein said optical passage is a core diameter expandingfiber.
 5. The optical filter module of claim 4, wherein an endface ofsaid optical passage exposed to a wall surface of said filter insertiongroove is a portion having an expanded core diameter of said corediameter expanding fiber.
 6. The optical filter module of claim 1,wherein a bottom of said filter insertion groove is formed to have ashape tapered from both groove wall surfaces toward a bottom surfacethereof.
 7. The optical filter module of claim 1, wherein a bottom ofsaid filter insertion groove is formed to have a V-shaped section. 8.The optical filter module of claim 1, wherein a bottom of said filterinsertion groove is formed to have a semicircular section.
 9. Theoptical filter module of claim 1, wherein a bottom of said filterinsertion groove is formed to have a shape tapered from one of groovewall surfaces toward the other.
 10. The optical filter module of claim1, wherein said filter is fixed along one of groove wall surfaces ofsaid filter insertion groove.
 11. The optical filter module of claim 1,wherein an angle formed by said optical passage with said filter rangesfrom 5 to 10 degrees.
 12. The optical filter module of claim 1, whereinthe angle formed by said optical passage with said filter is provided ona principal surface of said plane substrate.
 13. The optical filtermodule of claim 12, wherein the angle formed by said optical passagewith said filter is provided in a direction perpendicular to theprincipal surface of said plane substrate.
 14. The optical filter moduleof claim 1, wherein at least one of said plane substrate and said coversis fabricated to form a V-shaped groove thereon for receiving saidoptical passage.
 15. The optical filter module of claim 1, wherein saidcovers and said plane substrate are bonded together using one of aphoto-curing adhesive and a heat-hardening adhesive.
 16. A method ofmanufacturing an optical filter module comprising the steps of: forminga filter insertion groove so that said groove intersects an opticalpassage formed in a plane substrate; inserting a filter in said filterinsertion groove; moving at least one of two covers placed on said planesubstrate to sandwich and clamp therebetween and to support a portion ofsaid filter projecting from said filter insertion groove with saidcover, wherein a length of the portion of said filter projecting fromsaid filter insertion groove is at least half the thickness of each ofsaid covers; and bonding said cover onto said plane substrate.
 17. Themethod of manufacturing an optical filter module of claim 16 furthercomprising the steps of: forming a V-shaped groove on said planesubstrate; and forming said optical passage in said V-shaped groove. 18.A method of manufacturing an optical filter module comprising the stepsof: forming a V-shaped groove on a plane substrate; forming a filterinsertion groove so that said groove intersects said V-shaped groove;forming optical passages so that one end of each of said opticalpassages corresponds to said filter insertion groove; inserting a filterin said filter insertion groove; moving at least one of two coversplaced on said plane substrate to sandwich and clamp therebetween and tosupport a portion of said filter projecting from said filter insertiongroove with said cover, wherein a length of the portion of said filterprojecting from said filter insertion groove is at least half thethickness of each of said covers; and bonding said cover onto said planesubstrate.
 19. The optical filter module of claim 2, wherein saidportion extends beyond the tops of said covers.
 20. The method of claim16, wherein said portion extends beyond the top of said cover.
 21. Themethod of claim 18, wherein said portion extends beyond the top of saidcover.